Image-bearing photosensitive surfaces, such as prepress proofs and photographs, can be prepared using known techniques. Such photosensitive surfaces frequently have a protective coating on their surface to prevent or limit scratching, marring, or other imperfections in the image-bearing surface. The covering must be optically clear so as not to obscure or distort the image. It should also have flexibility, durability and good adhesion to the image-bearing photosensitive element.
Prepress proofs may be prepared from photosensitive materials that are either positive- or negative-working. A positive-working material is described in U.S. Pat. No. 3,649,268 to Chu and Cohen, and may contain tacky, photohardenable compositions which, when exposed imagewise to actinic radiation, harden in the exposed image areas, resulting in tacky and complementary non-tacky image areas. The image can be developed by applying colored particulate materials such as toners or pigments which selectively adhere to the unexposed, tacky image areas.
A negative-working process is described in U.S. Pat. No. 4,174,216 to Cohen and Fan, which teaches a negative-working element having a support; a tacky, nonphotosensitive contiguous layer; a photohardenable, photoadherent layer; and a strippable coversheet. After, imagewise exposure to actinic radiation, the coversheet is peeled away, carrying with it the exposed areas of the photoadherent layer, and leaving behind, the unexposed areas of the photoadherent layer which adhere to the tacky contiguous layer beneath. The areas of the tacky contiguous layer that do not contain the unexposed photoadherent layer, may then be toned with a finely divided particulate material. Different colored layers can be prepared and assembled in register over one another to form multilayer color proofs, as is well known to those skilled in the art. U.S. Pat. No. 4,053,313 to Fan describes a similar negative-working system which is developed by solvent washout.
Alternatively, a photosensitive imaging system may be (a) precolored with dyes and/or pigments or other coloring materials, (b) developed by washout solvents, and/or (c) contain photoplasticizing agents instead of photohardening agents. Other variations are known to those skilled in the art.
In addition to various photopolymer elements, other non-silver halide systems, e.g., diazo systems, are useful in preparing multicolor proofs.
Prepress color proofs such as those described above, may be covered with a protective covering or coversheet. One type of protective covering is exemplified in Cohen and Fan, U.S. Pat. No. 4,174,216. The patent describes a positive-working photosensitive element comprising a photosensitive layer and a polyester coversheet. The element is exposed overall to actinic radiation and the polyester coversheet is subsequently removed. Following subsequent development a proof with a desirable 20.degree. gloss level of about 65 gloss units is obtained. However, despite this desired gloss level, the proof has poor durability and is susceptible to crazing, scratching, and marring.
A second type of overcoat or protective covering is also described in Cohen and Fan, U.S. Pat. No. 4,174,216. The patent describes a negative-working element comprising a polyester cover sheet, a photosensitive layer, and an organic contiguous layer. The organic contiguous layer is laminated to the prepress proof and exposed overall with the polyester coversheet remaining intact. The result is a proof with a more durable finish, however, the gloss associated with the reflective, highly transparent, integral, polyester coversheet is unacceptably high, i.e., about 107 gloss units measured at 20.degree.. High gloss has long been recognized to be unacceptable by those skilled in the proofing art, and considerable effort has focused on ways to achieve a lower gloss, protective coating that is durable and resistant to crazing, scratching marring, etc.
A third type of protective coating is described in Bopp, U.S. Pat. No. 4,329,420. The coating consists of an integral layer of plastic film, such as polyester, that is coated with a nonphotoactive adhesive, and applied to the proof by lamination. This type of protective coating is indistinguishable from the second type of cover sheet discussed above in that the coating is durable, but exhibits unacceptable high gloss.
A fourth type of protective coating is described in Taylor, U.S. Pat. No. 5,019,536. This protective coating contains a thin and substantially transparent, nonphotosensitive, integral polymeric layer on a strippable substrate. The polymeric layer comprises a mixture of slightly incompatible polymers. In one embodiment, a wetting layer is provided on the side of the polymeric layer opposite the strippable substrate, and a covering film may be provided over the wetting layer. The protective coating provides the required gloss, however, having two slightly incompatible polymers increases the possibility for variability in coating solutions and therefore variability in the final product. Also, if the wetting layer and the covering film are present, unacceptable adhesion may occur. In other words, when the covering film is peeled off, part of the wetting layer and polymeric layer may peel off with it. Furthermore, the protective covering is incompatible with flammable solvents which is an environmental concern.
Accordingly, a need exists for a protective coating or overcoat which provides the necessary low gloss, the required adhesion to the strippable substrate and compatibility with flammable solvents. The element of the invention satisfies these requirements.